A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Known lithographic apparatus has a radiation source that produces radiation and an illumination system which conditions the radiation produced by the radiation source and directs a conditioned beam of radiation towards the patterning device. Some illumination systems have an illumination mode control arrangement that is capable of modifying the radiation beam to produce different illumination modes of the radiation beam. Each illumination mode of the radiation beam may have different characteristics, for example, angular intensity distribution and/or spatial intensity distribution. Each illumination mode may have a different angular intensity distribution in a pupil plane and a different spatial intensity distribution in a field plane. The illuminator may also have a uniformity control apparatus which selectively blocks parts of the radiation beam to control the uniformity of the radiation beam which is directed at the patterning device and hence the substrate. In some lithographic apparatus the uniformity control apparatus is used to make the scan-integrated intensity of the beam of radiation in the field plane (i.e., the spatial intensity distribution) as uniform as possible. The uniformity control apparatus may be in a plane which is not a field plane such that, by blocking parts of the radiation beam, it affects both the angular intensity distribution in a pupil plane as well as the spatial intensity distribution in a field plane. The substrate may be located in a field plane. In this case, if the spatial intensity distribution in the field plane is adversely affected by the uniformity control apparatus, then the imaging performance of the lithographic apparatus may also be negatively affected.
In some lithographic apparatus, a fixed grey filter is used to compensate for the change in the spatial intensity distribution in the field plane caused by the uniformity control apparatus. The fixed grey filter is configured such that it compensates for the change in the spatial intensity distribution in the field plane caused by the uniformity control apparatus for a particular illumination mode of the radiation beam produced by the illumination mode control arrangement. If the illumination mode of the radiation beam produced by the illumination mode control arrangement is changed, then the fixed grey filter will no longer be optimised to compensate for the change in the spatial intensity distribution in the field plane caused by the uniformity control apparatus. In this situation, because the fixed grey filter is not optimised to compensate for the change in the spatial intensity distribution in the field plane caused by the uniformity control apparatus, a reduction in the imaging performance of the lithographic apparatus may be caused. It is undesirable to remove the fixed grey filter and replace it with a fixed grey filter which is optimised for the new illumination mode, because to do so may be a time consuming and complicated procedure. Such a replacement of the fixed grey filter may therefore adversely affect the throughput (or production rate) of the lithographic apparatus. Furthermore, it may be impractical to have an optimised grey filter for every illumination mode of the lithographic apparatus.